The present invention is directed generally to flowmeters, and more particularly to an improvement in an acoustic flowmeter of the type which utilizes ultrasonic signals to determine the velocity of fluid flowing in a conduit.
Acoustic flowmeters of this type are generally known in the art. In accordance with conventional practice, a pair of ultrasonic transceivers are mounted to a fluid-carrying conduit at longitudinally spaced locations and facing each other diagonally across the conduit.. These transceivers or transducers are each excited at a known frequency by respective oscillators, such that the signal generated by the upstream transducer is received at the downstream transducer through the flowing fluid and vice-versa.
In accordance with practice in the prior art, a phase detector is utilized to compare the received signals and adjust the frequency of one of the oscillators such that the respective frequencies of the oscillators differ by a factor corresponding to the velocity of the fluid. The frequencies of the respective oscillators are then subjected to heterodyning, such that the algebraic sum and difference of the respective frequencies may be obtained. The algebraic sum and difference bears a known relationship to the velocity of the fluid, such that the velocity may be readily calculated therefrom. By providing a suitably calibrated meter and filter components to derive the algebraic sum and difference of the frequencies of the two oscillators, the velocity of the fluid may be displayed by the meter.
The foregoing prior art arrangement, while workable in theory, in practice has been found to suffer a serious problem of cross talk between the two channels. That is, the frequency signals generated by the respective oscillators and their associate transducers tend to become confused within the fluid medium, such that upon reception, considerable cross talk exists, making the two frequencies difficult to ascertain.
Alternative arrangements have been proposed for measuring the time of travel of a signal between respective transducers both upstream and downstream and deriving the velocity of fluid from the time difference. However, this arrangement requires extremely accurate measurement of exceedingly small time differentials and hence, accurate measurements have proven difficult to obtain. Moreover, such circuits also suffer from problems of turbulance in the conduit which tends to make the measured time differentials inaccurate.